In general, a flux used for soldering has any efficiency in chemical removal of a metal oxide that resides at a solder alloy and a metal surface of a joint object to be soldered and in movement of metallic elements at a boundary therebetween. Therefore, by performing the soldering using the flux, an intermetallic compound can be formed between the solder alloy and the metallic surface of the joint object to accomplish a strong joint.
The solder paste is formed by mixing the flux and the solder alloy powder. In a process of soldering by using the solder paste, a jointed object such as electronic parts or the like is mounted on a joint object such as a substrate, an electrode or the like to which the solder paste is applied, and then both the joint object and the jointed portion are heated in a reflow furnace. The solder alloy in the solder paste is melted by heating the solder paste and then is solidified to join the joint object with the jointed object.
In the soldering process with the reflow furnace, it is necessary to prevent a void from occurring in the solder alloy that is melted and solidified thereafter. A technology has been previously proposed to reduce the voids by setting a heating time to be longer during the solder melting period (see, for example, Patent Document 1).